As a multi-carrier transmission mode, an Orthogonal Frequency Division Multiplex (OFDM) technology converts a data stream transmitted in high-speed into a group of data stream transmitted parallel in low-speed, which makes the sensitivity of system to the multipath fading channel frequency selectivity reduce greatly. Moreover, the introduction of cyclic prefix further enhances the capability of anti-Inter-symbol Interference (ISI) for the system. In addition, features of high bandwidth utilization rate and simple implementation make the OFDM be applied in the field of wireless communication more widely. For example, a Long Term Evolution (LTE) system based on orthogonal frequency division multiplexing access and LTE-Advanced system, the next generation of LTE, are all systems based on the OFDM technology.
Relative to the LTE system, the LTE-Advanced system uses certain new technologies such as relay, spectrum polymerization and CoMP and so on to enhance system performance. The original intention of introducing the CoMP is to solve the problem of Inter-Cell Interference (ICI) in an OFDM system so as to improve the throughput of a cell-edge User Equipment (UE), e.g., multiple evolved Nodes B (eNBs) eliminate the ICI collaboratively and even change interference signals into desired signals. With the further progress of the research, it is discovered that applications of the CoMP in the LTE-Advanced can improve the data transmission rate, cell edge throughput and/or system throughput. Therefore, the overwhelming majority of companies are very bullish about the CoMP technology.
From a transmission direction, the CoMP technology can be divided into an Uplink (UL) CoMP technology and a Downlink (DL) CoMP technology. Wherein, the DL CoMP can be divided into two categories: Joint Process (JP) mode and Coordinated Scheduling (CS)/Coordinated Beamforming (CB) mode.
a) CS)/CB mode: only one node in multiple nodes sends data to the UE in an instant, which is similar to enhanced Inter-Cell Interference Coordination (ICIC), coordinated nodes do not share the data but share information related to channels, such as semi-static ICIC, coordinated Precoding Matrix Indicator (PMI) and coordinated beamforming and so on.
Wherein, in the coordinated scheduling/beamforming mode, the data sent to the UE are transmitted through a serving cell in which the UE is located; but a scheduling decision is under combination control, such as the control on the generation of interference in a coordinated cell and so on.
b) JP mode: multiple nodes send the data to one UE simultaneously, so as to improve the quality of received signals and/or eliminate the interference to other UEs.
The base stations and terminals participating in the CoMP coordination mainly include:
A serving cell: a cell sending control channels to the terminal. There is generally only one serving cell in the communication process between the terminal and the base station.
A measuring cell set: a cell set performing periodic channel state information measurement for the terminal. The set is semi-statically configured by the serving cell of the terminal.
A coordinated cell set: a cell set participating in sending service data to the terminal directly or indirectly.
Currently, with regard to the CoMP technology, the reached agreement mainly includes the following contents.
1. The Number of Cells in Participation should not be Excessive
The collaboration between multiple eNBs can improve the cell edge throughput rate and the average cell throughput rate in a large extent. However, sharing the data/Channel State Information (CSI) between multiple eNBs needs extremely large backhaul capacity, and thus the execution is very complicated. In order to reduce the complexity, it should be considered that the collaboration of a limited number of eNBs is used to serve specific UEs. Therefore, an issue related to the CoMP is how to select a coordinated cell cluster, so as to reach the maximization of cell throughput on the acceptable scheduling complexity and backhaul capacity.
2. The CoMP is Only Used for the Cell-Edge UE.
The CoMP can be used for the cell-edge UE and a cell-center UE theoretically, but a Signal to Interference plus Noise Ratio (SINR) of the cell-center UE itself is very high during non-CoMP. After the CoMP is used, the improvement of throughput is very tiny and information interaction is wasted. However, when the CoMP is used for the cell-edge UE, the inter-cell interference can be reduced evidently and the received power of the cell-edge UE can be enhanced. Therefore, most of companies consider that the CoMP should be only used for the cell-edge UE. The existing method includes selecting a coordinated UE according to the principle of SINR distribution, such as the coordination only to SINR<5 dB UEs.
3. It is Suitable for a Low-Speed UE
The multi-cell channel estimation can be optimized according to low-speed users. The feedback overhead of the low-speed users is small and the low-speed users can tolerate longer delay.
Currently, in the existing CoMP technology, a specific operating method for CoMP handover threshold is not considered. Therefore, the following problems mainly exist in the related art.
1) Since most of CoMP users are located at cell edges, the number of users located at the handover positions account for about 5% and the number of users located at the CoMP account for about 30% according to the analysis, excessive signaling overhead is caused if all the users report measurement sets frequently.
2) If simple Reference Signal Received Power (RSRP) threshold handover algorithms such as Abs(RSRPcell−RSRPsource)dB<RSRPthresholdin and Abs(RSRPcell−RSRPsource)dB>RSRPthresholdout are used, frequent in-set and out-set operations are easily triggered as the definition of thresholds is limited.